This invention relates to torque transmissions and more particularly, it concerns improvements in continuously variable ratio transmission units by which the ratio range of such units is increased over the ratio range of presently available units without compromise in operating efficiency and power density.
Various embodiments of continuously variable transmissions in which the present invention is particularly useful are disclosed in U.S. Pat. Nos. 4,112,779 and 4,112,780, both issued Sept. 12, 1978 and in 4,152,946 issued on May 8, 1979. In the type of transmission exemplified by these patents, three frame supported working bodies operate to transmit a mechanical power input to a rotatable output at continuously variable output/input speed ratios within the design range of the transmission. For purposes of definition in this background discussion as well as in the ensuing detailed description of the present invention and in the appended claims, the three working bodies may be termed respectively, an "alpha body" which is supported by the transmission frame to be concentric with a first axis, a "beta body" which is concentric with a second axis inclined with respect to and intersecting the first axis at a point of axes intersection, and an "omega body" carried by or forming part of the frame to be concentric also with the first axis. Although any one of these three bodies may be rotatable on the respective axes with which they are concentric, one of the three is held against rotation to provide a reaction torque whereas the other two bodies are rotatable and coupled either directly or by gearing to the respective input and output shafting of the transmission.
The capability for the continuously variable speed ratio in such transmissions is achieved by providing one of the beta and omega bodies with a pair of rolling or traction surfaces which are surfaces of revolution about the concentric body axis and which are of variable radii along that axis in symmetry with the point of first and second axes intersection. Physically, such rolling surfaces will be provided by conical or cone-like members. The other of the beta and omega bodies is provided with a pair of rolling or traction surfaces of revolution about the concentric body axis but which are of relatively constant radius. The pairs of rolling surfaces on the beta and omega bodies are retained in frictional engagement with each other at two contact points or zones capable of positional adjustment to vary the ratio of the beta body surface radius (R.sub.b) to the omega body surface radius (R.sub.w). Thus, if the alpha body is rotatable at a velocity (.alpha.) about the first axis, the rotational speed of the beta body about the second axis in a fixed frame of reference is (.beta.) and the rotational speed of the omega body on the first axis is (.omega.), then the respective speeds of the three bodies are related by the following equation: EQU .omega.-.alpha.+(.alpha.-.beta.) R.sub.b /R.sub.w =0. (1)
A generally preferred mode of operating such transmissions has been to apply an input torque to the alpha body to carry the beta body in nutation and hold the omega body against rotation (.omega.=0). The beta body is linked with an output shaft rotatable on the first axis by gearing having a ratio factor (k) which theoretically may be of any value and also may be made either positive or negative depending on the particular gearing arrangement used. In light of the foregoing, where .theta. is unit output speed and taking into account the gearing ratio (k), the output/input speed ratio of the unit is determined by an equation: EQU .theta./.alpha.=1-kR.sub.w /R.sub.b. (2)
If, for convenience, the function R.sub.w /R.sub.b is designated as a radius ratio or (.rho.), then Equation (2) becomes: EQU .theta./.alpha.=1-k.rho.. (3)
The performance characteristics of such transmissions are described in an article entitled: "Performance of a Nutating Traction Drive" by P. Elu and Y. Kemper, paper no. 80-C2/DET-63, the American Society of Mechanical Engineers. In this Article, it is noted that extremely high overall efficiencies are possible by appropriate selection of the gear ratio factor (k) though with a corresponding reduction in the transmission speed ratio range (e.g., 2.8/1.9 or 1.5 vs. 0.43/0 or .infin.). The Article also makes reference to a "power multiplication factor" which results from the epicyclic motion of the nutating beta body or member and which may be visualized as variation in the power "seen" at the points of rolling friction engagement between the beta and omega bodies for a given power input. Also, the term "power density" is used in the Article to express the power transmitting capacity of a particular transmission unit for a given input speed.
Heretofore, all embodiments of transmissions of the type disclosed in the aforementioned U.S. patents or "nutating traction drives" have involved a direct connection of one of the unit input and unit output to a first of the alpha, beta or omega bodies, a retention of a second of such bodies as a reaction member and preferably a gearing linkage (with a gear ratio k) of the other of the unit input and the unit output to the third of such bodies. As a result, the attainment of high overall efficiencies and increased power densities was possible only with a severe curtailment of speed ratio range. In this respect, it should be noted that the radius ratio or (.rho.) in Equation (3) is variable within limits dictated by the size and geometry of the transmission.
It is apparent, therefore, that continuously variable transmissions of the type mentioned are capable of achieving high operating efficiencies. It is equally apparent, however, that a need exists for expanding the speed ratio range of such transmissions without compromise of operating efficiency or of high power densities.